In recent years, there are drastically progressing developments of organic electroluminescent (organic EL) devices which are self-luminous devices with electrons and holes as carriers. Organic EL has features of being capable of achieving more thickness reduction and weight reduction and being better in visibility than liquid crystal devices, which necessitate backlights and are non-self-luminous.
The organic EL devices usually have a pair of substrates on surfaces facing each other of which respective electrodes are formed, and a light-emitting layer disposed between the pair of substrates. Among these, the light-emitting layer is composed of an organic thin film containing a light-emitting material to emit light by application of a voltage. When such organic EL devices are made to emit light, holes and electrons are injected by applying a voltage from an anode and a cathode to the organic thin film. Thereby, light emission can be obtained due to that holes and electrons are caused to be recombined in the organic thin film and excitons produced by the recombination return to their ground state.
In the organic EL devices, in addition to the light-emitting layer, a hole injection layer and an electron injection layer to raise the injection efficiency of holes and electrons, and a hole transport layer and an electron transport layer to improve the recombination efficiency of holes and electrons, respectively, must be provided between the light-emitting layer and the electrodes. Hence, the organic EL devices come to have a multilayer structure, making the structure complex and increasing the number of the production processes. Further the organic EL devices have many restrictions, since the work functions must be taken into consideration in selection of electrode materials to be used for anodes and cathodes.
As self-luminous devices coping with these problems, light-emitting electrochemical cells (LECs) have recently attracted attention (Patent Literatures 1 and 2). The light-emitting electrochemical cells generally have a light-emitting layer containing an ionic compound and a light-emitting material. As the ionic compound, various types of inorganic salts and organic salts are used; and as the light-emitting material, organic polymers, metal complexes and the like are used. In the voltage application, cations and anions originated from the ionic compound migrate in the light-emitting layer toward a cathode and an anode, respectively, and make large electric field gradients (electric double layers) at electrode interfaces. Since the formed electric double layers facilitate injection of electrons and holes at the cathode and anode, respectively, the light-emitting electrochemical cells have no need of having a multilayer structure as in organic EL. Further since the work functions of materials to be used as cathodes and anodes are not required to be taken into consideration for the light-emitting electrochemical cells, there are few restrictions on the materials. For these reasons, the light-emitting electrochemical cells are expected as self-luminous devices capable of reducing the production cost more largely as compared with the organic EL.